Robust Superhydrophobic Surfaces via the Sand-In Method.

It is necessary to theoretically research wettability in superhydrophobic surface fabrication. Here, a numerical calculation approach is proposed for determining the contact angle of the water droplets on array micropillars by wire electrical discharge machining (WEDM). A hierarchical model is employed for these array microstructures, including mechanical analysis for a water droplet placed on a smooth array and wettability evaluation on the morphology of the WEDM surface. On pillars, equations are listed to solve the apparent contact angle according to force balance of gravity, tension, and pressure. As for the WEDM morphology, temperature simulation and measurement are carried out, and then the effect of roughness on surface wettability is studied. Constructed formulas predict the contact angle, and then the effect of geometric dimensions is obtained. In order to verify the assumption, array micropillars with different cross-profiles are prepared using high-speed WEDM on the Al alloy surface. Through the results of contact angle determination, the numerical calculation is carried out. This theoretical prediction is beneficial for improving the fabrication of the superhydrophobic surface by WEDM.

Tuning wettability and getting superhydrophobic surface by controlling surface roughness with well-designed microstructures

Superhydrophobic micropillar surfaces, inspired by the lotus leaf, have been extensively studied over the past two decades for their self-cleaning, anti-friction, anti-icing, and anti-corrosion properties. In this study, we introduce a simple and effective method for introducing porosity into polydimethylsiloxane (PDMS) micropillar arrays using salt templating. We then evaluate the wetting behaviour of these surfaces before and after infusion with perfluoropolyether (PFPE) oil. Apparent contact angle and sliding angle were measured relative to a non-porous control surface. Across five porous variants, the contact angle decreased by approximately 5° (from 157° to 152° on average), while the sliding angle increased by about 3.5° (from 16.5° to 20° on average). Following PFPE infusion, the porous arrays exhibited reduced sliding angles while maintaining superhydrophobicity. These results indicate that introducing porosity slightly reduces water repellency and droplet mobility, whereas PFPE infusion restores mobility while preserving high water repellency. The change in wettability following PFPE infusion highlights the potential of these surfaces to function as robust, self-cleaning materials.

The core issue in the theoretical design of superhydrophobic surfaces is to elucidate the relationship between surface microstructure and wettability. In this paper, four two-scale array microstructure models with different shapes are proposed to predict the apparent contact angles (CAs) of EN-GJL-250 (Grey Cast Iron) surface under different wire-cut electrical discharge machining (WEDM) discharge parameters. The primary microstructure is the artificially designed semicircle, rectangular, sawtooth and square column array microstructure, and the secondary microstructure is the micro-nanostructure of the surface machined by WEDM. Firstly, the mechanical analysis of water droplets placed on the primary array microstructure is carried out by the force balance method. According to Newton’s third law, equations containing CA parameters are listed. Secondly, the finite element method (FEM) was used to analogue simulation the distribution of material surface temperature and flow field in the WEDM single-pulse discharge machining process. The surface equivalent geometric model of WEDM was established by a uniform arrangement of single pulse pits obtained by simulation, the surface roughness coefficients under different discharge parameters were calculated, and the influence of the surface roughness coefficients on the surface wettability was studied. The numerical results show that the CA is jointly determined by the surface roughness coefficient under different discharge parameters, and the shape and size parameters of the microstructure. Finally, semicircle, rectangular, sawtooth and square column array microstructures were fabricated on EN-GJL-250 surface by high-speed WEDM. Experimental results show: The CA of water droplets on the square column array microstructure is the largest, the predicted values of the CA were in good agreement with the experimental values, and the average relative error was 7.83%.

It is well known that heat transfer in dropwise condensation (DWC) is superior to that in filmwise condensation (FWC) by at least one order of magnitude. Surfaces with larger contact angle (CA) can promote DWC heat transfer due to the formation of “bare” condensation surface caused by the rapid removal of large condensate droplets and high surface replenishment frequency. Superhydrophobic surfaces with high contact angle (> 150°) of water and low contact angle hysteresis (< 5°) seem to be an ideal condensing surface to promote DWC and enhance heat transfer, in particular, for the steam-air mixture vapor. In the present paper, steam DWC heat transfer characteristics in the presence of noncondensable gas (NCG) were investigated experimentally on superhydrophobic and hydrophobic surfaces including the wetting mode evolution on the roughness-induced superhydrophobic surface. It was found that with increasing NCG concentration, the droplet conducts a transition from the Wenzel to Cassie-Baxter mode. And a new condensate wetting mode—a condensate sinkage mode—was observed, which can help to explain the effect of NCG on the condensation heat transfer performance of steam-air mixture on a roughness-induced superhydrophobic SAM-1 surface.

Icephobicity and durability assessment of superhydrophobic surfaces: The role of surface roughness and the ice adhesion measurement technique

Dynamic behavior of water droplets impacting on the superhydrophobic surface: Both experimental study and molecular dynamics simulation study

Steam condensation with non-condensable gas exists widely in petro-chemical, power plant and seawater desalination processes. Super-hydrophobic surfaces with high contact angle could be an promising condensing surface to promote DWC and enhance heat transfer. As pure steam condensing droplet completely occupies micro/nano-structures of super-hydrophobic surfaces, super-hydrophobic function becomes “invalid”. For stream-air mixture vapor, the non- condensable gas would occupy the micro/nano-structures instead of water. Droplets form Cassie or transition wetting mode, holding or partly holding super-hydrophobic feature. Inclination angle of the tube can influence the shape, size and dynamic characteristics of the condensing droplets, thus changing the heat transfer performance. Therefore, for getting deeply insights of the effect of the non-condensable gas on condensation heat transfer, it is necessary to study the condensing drops characteristics on super-hydrophobic tube for different inclined angles. According to dropwise condensation model, increasing contact angle hysteresis, departure droplet size, sweeping period would reduce the condensation heat transfer efficiency. In this paper, dropwise condensation is performed on super-hydrophobic copper tube, departure droplet size, sweeping period and the phenomenon of contact angle hysteresis are thoroughly studied and are compared with the case of hydrophobic surface or pure steam to explore the influence of inclined angle and surface wettability on droplet dynamics to achieve condensation heat transfer enhancement. The experimental result shows: (1) On the top of copper tube, for the condensation of steam with 10% non-condensable gas, the contact angle hysteresis on super-hydrophobic surface is smaller than that on hydrophobic surface. Compared with pure steam condensation on super-hydrophobic surface, the contact angle hysteresis of mixed steam is smaller because the non- condensable gas fills the micro/nano-structures. However, on the middle and bottom parts of the tube, the contact angle hysteresis on super-hydrophobic surface is greater than that on hydrophobic surface for the condensation of both pure steam and mixed steam. This phenomenon indicates that the contact angle hysteresis is determined by surface wettability instead of inclination angle. (2) The departure droplet size on super-hydrophobic surface increases with the inclination angle and the opposite trend is found on hydrophobic surface. With the increase of inclination angle, the support force provided by the tube would increase and change direction. So the wettability of the surface and departure droplet size would be changed. (3) The sweeping period of droplet on super-hydrophobic surface is longer because the micro/nano-structures increase the adhesion between the surface and the droplets. With the increase of the inclination angle, the sweeping period decreased slightly, shorter than the sweeping time when the tube is vertical. The sweeping period of vapor with non-condensable gas condensation is greater than that of pure steam. The presence of non-condensable gas increases thermal resistance, thus slowing down the drop growth rate and increasing the sweeping period. To sum up, on the top of copper tube, the contact angle hysteresis on super-hydrophobic surface is smaller than that on the hydrophobic surface. However, the totally opposite trend is found on the middle and bottom parts of the tube. The departure droplet size on super-hydrophobic surface increases with the inclination angle. The sweeping period of droplet on super-hydrophobic surface is longer. This work offers a new avenue to further enhance heat transfer of condensation with non-condensable gas.

THE large values given by Fogg1 for the contact angles of water with leaves suggest that these are apparent rather than true contact angles. Adam2 and Wenzel3 have shown that rough surfaces give an apparent contact angle which is greater than the true contact angle for the smooth material of the surface when the true contact angle is greater than 90°. We have recently extended this theory to porous surfaces4, and to surfaces so rough that much air is entrapped at the interface between the water and the solid; large apparent contact angles are then possible when the true contact angle is even less than 90°. The apparent contact angle is given by where θD is the apparent or observed contact angle, θ is the true angle, f1 is the area of solid – water contact and f2 is the area of air – water contact per unit superficial area of the interface.

Spread-Wing Postures and the Water Repellency of Feathers: A Test of Rijke's Hypothesis

Inspired by nature, superhydrophobic surfaces have been widely studied. Usually the wettability of a superhydrophobic surface is quantified by the macroscopic contact angle. However, this method has various limitations, especially for precision micro devices with superhydrophobic surfaces, such as biomimetic artificial compound eyes and biomimetic water strider robots. These precision micro devices with superhydrophobic surfaces proposed a higher demand for the quantification of contact angles, requiring contact angle quantification technology to have micrometer-scale measurement capabilities. In this review, it is proposed to achieve micrometer-scale quantification of superhydrophobic surface contact angles through droplet adhesion characteristics (adhesion force and contact radius). Existing contact angle quantification techniques and droplet characteristics' measurement methods were described in detail. The advancement of micrometer-scale quantification technology for the contact angle of superhydrophobic surfaces will enhance our understanding of superhydrophobic surfaces.

Fabrication of robust superhydrophobic surface on TC4 alloy by wire electrical discharge machining

The liquid phase response of quartz crystal microbalances (QCMs) with a thin coating (∼9 µm) of epoxy resin with and without a carbon nanoparticles top layer is reported. The nanoparticles convert the epoxy surface to a superhydrophobic one with a high static contact angle (∼151°–155°) and low contact angle hysteresis (∼1°–3.7°) where droplets of water are in the suspended Cassie–Baxter state. The frequency decrease of the fully immersed QCM with the superhydrophobic surface is less than with only epoxy layer, thus indicating a decoupling of the QCM response. A wettability transition to a liquid penetrating into the surface roughness state (for droplets a high contact angle hysteresis Wenzel state) was triggered using a molarity-of-ethanol droplet test (MED) and electrowetting; the MED approach caused some surface damage. The electrowetting-induced transition caused a frequency decrease of 739 Hz at a critical voltage of ∼100 V compared to the QCM in air. This critical voltage correlates to a contact angle decrease of 26° and a high contact angle hysteresis state in droplet experiments. These experiments provide a proof-of-concept that QCMs can be used to sense wetting state transitions and not only mass attachments or changes in viscosity–density products of liquids.

An outlook on tunable superhydrophobic nanostructural surfaces and their possible impact on ice mitigation

Stable superhydrophobic si surface produced by using reactive ion etching process combined with hydrophobic coatings